1. Field of the Invention
The present invention relates to an apparatus for applying a bias magnetic field used in a magnetooptic disk drive unit, etc.
2. Description of the Related Art
In a magnetooptic recording operation, an erasing magnetic field is applied to a magnetooptic disk while a laser beam having recording power is normally irradiated continuously onto the magnetooptic disk. Thus, a magnetizing direction is arranged in one direction in the magnetooptic disk. Next, the laser beam having recording power and modulated in accordance with recording information is irradiated onto the magnetooptic disk while a recording magnetic field having a direction opposite to that of the erasing magnetic field is applied to the magnetooptic disk. The above two operations are required to record and erase information. Accordingly, it is necessary to switch the erasing and recording magnetic fields as soon as possible so as to shorten the time for recording information.
In general, as a method for shortening the time for performing such a switching operation, there are proposed methods in which a magnet having a rectangular shape and arranged in parallel with the magnetooptic disk is rotated at recording and erasing times of information to change S and N polar directions of the magnet facing the magnetooptic disk.
Japanese Utility Model Application Laying Open (KOKAI) No. 64-42503 shows an apparatus for applying a magnetic field and constructed in accordance with the above proposed method.
However, in such an apparatus, part of the torque provided by coils in positions opposite to a magnetic pole are applied to the magnet in a direction for preventing the rotation thereof when the magnet begins to be rotated. Such a part of the torque is also applied to the magnet in a direction for rotating the magnet when the rotation of the magnet is stopped. Accordingly, part of the electric current flowing through the coils is not effectively used. Therefore, the time required to rotate the magnet 180.degree. is increased and the erasing and recording magnetic fields cannot be rapidly switched on a magnetooptic recording face.